Or maybe you just want to find where there are duplicates, or filter out the unique records.

-- Find all records with only one occurence.SELECT email
FROM users
GROUPBY email
HAVING ( COUNT(email) = 1 )
-- Search for records with more than one occuranceSELECT email,
COUNT(email) AS NumOccurrences
FROM users
GROUPBY email
HAVING ( COUNT(email) > 1 )